Honestly, I like there being multiple denominations of currency as it makes a world feel alive, and if it is simple as DnD denominations are, then it won't be problematic. The game could in the buying and selling screen tally up the value of your currency, it could all be stored in a money pouch so it could be both handled like an item that you can remove, and like currency used to buy an sell. And change can be automatically handled so say you maybe have 100 gold but try to buy a one copper item, you would end up with 99 gold and 99 copper (or 99 gold, 9 silver, and 9 copper) and your currency screen could display it in a simple way of 99.99 gold with the option to display it more like dragon age which would display the gold silver and copper separately (but dragon age would automatically round it up so if you had 100 silver it became 1 gold).
